Recently, the pneumatic radial tire is increasingly flattened in accordance with the speeding-up floor-lowering of vehicles, whereby it is tended to increase a radially size growth quantity of a tread portion through the filling of an internal pressure. Such an increase of the radially size growth quantity produces a separation failure at both widthwise end portions of a belt layer to deteriorate a belt durability. In order to suppress the size growth in the tread portion, particularly in the vicinity of a shoulder portion through the filling of the internal pressure, a tire wherein the belt layer is reinforced with a belt reinforcing layer embedded with reinforcing elements extending in a circumferential direction has been proposed, for example, in JP-A-2-208101.
As shown in FIG. 1, this tire comprises a belt layer 114 arranged around a carcass layer 111 in a tread portion 112 and comprised of, for example, two cross belt plies 113a, 113b embedding many reinforcing elements therein crossed with each other with respect to an equatorial plane S of a tire at an inclination angle of 10-40 degrees with respect to the equatorial plane S, and a belt reinforcing layer 116 arranged between the belt layer 114 and the carcass layer 111 and comprised of at least one reinforcing ply embedded with reinforcing elements extending along the equatorial plane S and having a width narrower than that of the widest-width belt ply 113a, two reinforcing plies 115a, 115b in the figure.
As the flattening of the tire is further promoted to become an aspect ratio of not more than 0.70, the above side growth can not be effectively suppressed by the belt reinforcing layer 116 having such a conventional width. For this end, it has been proposed to suppress the side growth by widening the width of the widest-width reinforcing ply in the belt reinforcing layer 116 to not less than 0.6 times a section width W of the tire while rendering into less than the width of the widest-width belt ply 113a. In the latter tire, however, there is caused a problem that an interlayer separation is produced between each widthwise outer end portion of the belt reinforcing layer and the belt layer to deteriorate the belt durability.
Now, the inventor has made various studies with respect to the aforementioned interlayer separation and found that interlayer shearing deformation is a cause of producing the interlayer separation. That is, the belt layer 114 and the belt reinforcing layer 116 in a ground contact region are deformed from an arc shape to a flat shape during the contacting of the tire with ground viewing from a side face of the tire, and as a result, the elongation in the circumferential direction is produced in both the layers and the widths of the belt layer 114 and the belt reinforcing layer 116 are reduced by such an elongation. Since the reinforcing elements embedded in the belt reinforcing layer 116 extend substantially in the circumferential direction, the widthwise rigidity of the layer is approximately equal to the rigidity of rubber itself, and as a result, when the elements elongate in the circumferential direction as mentioned above, the belt reinforcing layer 116 is largely reduced in the widthwise direction.
On the contrary, since the reinforcing elements embedded in the belt plies 113a, 113b of the belt layer 114 are crossed with each other with respect to the equatorial plane S of the tire, these reinforcing elements are bridged to render the widthwise rigidity of the belt layer 114 into a higher value. Therefore, when the layer is elongated in the circumferential direction as previously mentioned, the widthwise reducing quantity of the belt layer 114 takes a value fairly smaller than that of the belt reinforcing layer 116, and as a result, the shearing deformation is produced between the belt layer 114 and the belt reinforcing layer 116 in the widthwise section of the tread portion.